Treatment strategies for Muscular Dystrophies will be improved by deepening understanding of how muscle formation is controlled and encouraged. The much needed identification and dissection of underlying molecular pathways is readily advanced through studies in model systems. This application focuses on investigating the mechanisms that facilitate the muscle forming process of myoblast fusion. Drosophila is used as the model system. Drosophila's Hibris (Hbs) and Sticks and stones (Sns) are transmembrane receptor-like proteins present on myoblasts. They are structural homologues. They are not functionally interchangeable. They are essential for successful myoblast fusion. No other model system or molecules provide such compelling, accessible entry points for dissecting the molecular processes supporting myoblast fusion. In this application, experiments are proposed in Specific Aim 1 to investigate how Hbs and Sns interact with a common downstream molecular target. The interactions will be assayed in vivo at the protein, genetic and cellular level through this aim. Specific Aim 2 capitalizes on observations that structural differences in the Hbs and Sns endodomains translate into differences in Hbs and Sns behaviors. The functions of the differing endodomain regions will be investigated using deletion and chimeric constructs in cellular and in vivo assays. Specific Aim 3 expands upon observations that Hbs and Sns are endocytosed, and that Hbs and Sns trans-endocytose a common extracellular ligand. Endocytosis takes Hbs and Sns from the membrane to the endosomes. Ligand trans-endocytosis by Hbs and Sns involves simultaneous transfer of membrane from the ligand-expressing cell, ligand internalization, and ligand re-presentation at the Hbs and Sns expressing cell's membrane. It is postulated that endoctyosis regulates Hbs and Sns levels at the membrane, while ligand trans-endocytosis mediates signal propagation and membrane destabilization. The putative functions for Hbs and Sns endocytosis and trans-endocytosis will be tested in vivo in this aim. In addition, molecular candidates will be tested in cell culture and in vivo for their ability to regulate Hbs and Sns endocytosis and trans-endocytosis. In summary, Hbs and Sns are key molecular players involved in signaling, endocytosis and trans-endocytosis in fusing myoblasts. Elucidating their actions and interactions will deepen our understanding of the roles these proteins and processes play in successful myoblast fusion. [unreadable] [unreadable] [unreadable]